An electrophotographic image forming apparatus has been commercialized not only as an apparatus for forming a monochromatic image, but also, as an apparatus for forming a color image. As the electrophotographic image forming apparatus has come to be used in various fields, the level of image quality required to be produced by an electrophotographic image forming apparatus has increased. More specifically, the level of image quality which matches in graininess and glossiness the level of image quality of an image formed by silver salt photography has come to be demanded. One of the technologies for obtaining a color image of excellent glossiness involves the transferring of a color toner image onto a sheet of a recording medium provided with a transparent resin layer formed of thermoplastic resin, and then fixing the transferred toner image to the recording medium to yield a copy which is flat and smooth across the surface.
More specifically, one of the above described image formation methods is disclosed in Japanese Laid-open Patent Application 2004-118020. According to the image formation method described in this publication, an unfixed toner image borne on a sheet of a recording medium is fixed by a heat roller (first fixing apparatus). Then, the sheet of the recording medium is again subject to heat and pressure by a fixation roller (second fixing apparatus), with a fixation belt placed between the fixation roller and the sheet of the recording medium. Then, the sheet of the recording medium is separated from the fixation belt after the sheet is cooled.
As a result, the toner image is fixed while remaining inlaid in the transparent resin layer of the sheet of the recording medium. During the fixation operation, both the surface of the transparent resin layer of the recording medium and the surface of the toner image solidify while conforming to the surface of the fixation belt. Therefore, the sheet of recording medium becomes flat and smooth across its surface, inclusive of the surface of the toner image, thereby yielding a color image of excellent glossiness.
However, the above-described image formation method suffers from the following problem. That is, when the color toner image is fixed by a heat roller during the aforementioned first stage of fixation, the toner layers (which make up the color toner image) are squashed, and as they are squashed, they tend to spread in a direction parallel to the surface of the sheet of the recording medium. Once the toner layers spread in the above-mentioned direction, the image that is produced thereby is inferior in terms of graininess, even if the toner layers are inlaid into the transparent resin layer of the sheet of the recording medium.
In other words, the graininess of the toner image undesirably decreases during the first stage of fixation, although whether or not the quality of the toner image decreases depends on the conditions under which the toner image is fixed during the first stage of fixation. Therefore, it is impossible to obtain a highly glossy image, which is as high in quality as an image formed with the use of silver salt photography, with the use of the abovementioned method.